Development of Copper Telluride based Thermoelectric Materials Synthesis, Microstructure and Properties

Abstract

Thermoelectric materials play an important role in harvesting the waste heat into useful electrical energy. Since thermoelectricity can lead to a clean energy conversion technology, it is important to find suitable TE materials. The state-of-the-art thermoelectric materials comprises mostly of tellurides due to their favourable transport properties. In this thesis, the structural and thermoelectric properties of copper telluride based alloys have been explored. Copper telluride belongs to the “Phonon Liquid Electron Crystal (PLEC) class of materials owing to its mobile Cu ions which migrates easily inside the crystal lattice. The “mobile” Cu ions restrict the propagation of transverse phonon vibrations during heat transfer. This facilitates the reduction of thermal conductivity making it a suitable choice as thermoelectric material. However, formation of copper vacancies results in high hole carrier concentration (~ 1021 cm-3), which degrades the thermoelectric properties and long-term stability of Cu2Te. Moreover, the crystal structure corresponding to the different phases in Cu-Te is still ambiguous. This pose challenges in proper understanding of the properties of the Cu-Te alloys. Therefore, in the first part of the thesis, the formation of different crystal structures with changes in stoichiometry between Cu and Te has been investigated. The increase in carrier concentration with decrease in Cu:Te stoichiometry was observed. High carrier concentration led to poor power factor and lower thermoelectric figure of merit (zT). It was concluded that in order to enhance the thermoelectric performance of Cu2Te, the carrier concentration has to be decreased by compensating the copper vacancies. In the second part of the thesis, Cu2Te was alloyed with Fe to tune the charge carrier concentration. Substitution of trivalent Fe at the monovalent Cu site resulted in the compensation of hole-carrier concentration and led to an enhancement in power factor. The ‘liquid-like’ mobility of Cu ions was also suppressed as confirmed from the higher specific heat capacity values. The figure of merit was improved for the Fe alloyed samples. The final part of the thesis deals with further improving the thermoelectric properties of Cu2Te by alloying with Sb2Te3. The increase in the Sb2Te3 phase content helped in reducing the electrical as well as thermal conductivity of Cu2Te. This in turn enhanced the figure of merit to ~0.6 at 600 K. Till date, Sb2Te3 is known to be an effective thermoelectric material near room temperature. However, the results obtained herein shows that the high temperature zT as well as the mechanical strength of Sb2Te3 can also be improved in such a composite system of Cu2Te-Sb2Te3